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As there was no way to make sure he would remain alive long enough for her to finish her last mission if left as he was, Emilia naturally couldn't leave Amos in the hands of Ginny and Jasper.
With the grudge that the twins now sported against the boy, it wouldn't be strange if they drained him dry of every drop of blood in his body, and only a fool would expect them to get Amos the medical attention he needed to survive.
Of course, Emilia could just give him a drop of her blood or instruct the twins to keep him alive with her 'real identity', but that would involve revealing her 'involvement' and 'special interest' in Amos to outsiders, which would just be troublesome.
Needless to say, even if they hadn't yet discovered his real identity, Emilia wasn't stupid enough to believe they wouldn't be able to dig it up if she left the boy with them.